The present invention relates to a network for reproducing a clock signal from the baseband digital signal demodulated at a PCM receiver.
In a PCM microwave communication system, pulse code modulated carrier waves are demodulated at the receiving end into a baseband signal. The demodulated digital signal is applied to the code converter, decoder and remodulator of the relaying transmitter or the like which follow the demodulator stage. In these circuits which follow the demodulator, the demodulated digital signal must be synchronized with a clock signal reproduced synchronously from the digital signal. Generally, the digital signal representing the information is constructed of a digital code train with marks and spaces, with the clock signal component lying in the transition between a mark and a space. In practice, the mark and the space do not always occur alternately; often, the mark or the space occurs serially. In such duration of a mark or a space, it is virtually impossible to reproduce the clock signal component from the digital signal. One solution to this problem is to use a bandpass filter of narrow bandwidth or a phase-locked oscillation circuit to make it possible to reproduce the clock signal for the duration of the pulse code train which holds no clock signal component. In the system using the bandpass filter, the narrower the bandwidth, the greater will become the delay time in response at the output. Widening the bandwidth of the filter to reduce the delay time in output response will result in increase in jitter, causing the reproduced clock signal to be ruined. While, in the system using the phase-locked oscillation circuit, it takes a considerable time to establish the synchronized state.
In a communication network in which a number of stations communicate with each other by way of a common relay medium on a time-division basis, as in the TDMA satellite communication system which is expected to be put into practical use in near future, the digital signals transmitted from the individual stations are intermittent, so-called "burst signal" and also, asynchronization is present between the individual burst signals. At the receiving end, therefore, the respective clock signals must be reproduced independently of each other for individual bursts. Furthermore, if an extra time is taken at the receiver end for the reproduction of the clock signal at the beginning of each burst, this will make it difficult to retime the received digital signal at the beginning of the burst, resulting in the reduction of the efficiency in such communication network. After all, the use of the bandpass filter of narrow bandwidth or the phase-locked oscillation circuit is not an efficient answer to the problem involved in pulse code communications based on the TDMA system.